The present invention relates to a novel non-linear resistance element or, more particularly, to a novel resistance element with very stable performance which is a sintered body comprising, as the essential components, titanium dioxide, bismuth oxide and a third component.
The invention also provides an improved method for the preparation of the above described non-linear resistance elements and noise suppressors with the above non-linear resistance element.
The invention further provides a noise suppressor utilizing the non-linear resistance element above suitable to eliminate the noise voltage generated, for example, in miniature motors built in various precision electronic instruments.
The rapid development and growth in the fields of audio instruments, controlling instruments and small-sized rotary machines such as small or miniature motors in recent years have presented important problems in suppressing the noise voltage generation from the motors, protection of the instruments or motors from over-voltage and protection of contact points in relays. For solving such problems, so-called varistor elements or non-linear resistance elements, i.e. elements having markedly non-linear volt-ampere characteristic, are essential as a component of the circuit. The non-linear resistance elements must naturally satisfy diversified requirements in the performance thereof along with the requirement of low cost for the production thereof in consideration of the relatively low prices of the instruments or motors.
Various types of non-linear resistance elements have hitherto been proposed for satisfying the above requirements which sometimes conflict with each other. Several of the nonlinear resistance elements typically known in the art are made of silicon carbide-based sintered bodies, selenium or cuprous oxide varistors, zinc oxide-based sintered bodies and the like.
Needless to say, the most important characteristic parameter in varistor elements is the so-called non-linearity index .alpha. which is related to the voltage-current characteristic as expressed by the equation EQU I=(V/C).sup..alpha.,
where V is the voltage applied to the varistor element, I is the current across the varistor element, C is a constant corresponding to the voltage with a predetermined current and .alpha. is the nonlinearity index defined by the equation EQU .alpha.=log.sub.10 (I.sub.2 /I.sub.1)/log.sub.10 (V.sub.2 /V.sub.1),
in which V.sub.1 and V.sub.2 are the voltages with given current I.sub.1 and I.sub.2, respectively,
This value of non-linearity index .alpha. is taken as a measure for evaluating the performance of non-linear resistance elements and, when .alpha. is equal to 1, i.e. I.sub.2 /I.sub.1 =V.sub.2 /V.sub.1, the element is an ordinary ohmic resistor element and larger values of .alpha. are usually preferred for most of the varistor elements. Further, preferred values of the constant C depend on the use of the varistor element but relatively low C-values are recommended for varistors to be used at low voltages although it is a general requirement that any desired C-values can readily be obtained according to need.
Among the known varistor elements hitherto in use, those of silicon carbide-based sintered body are prepared by sintering silicon carbide particles of about 100 .mu.m diameter with clay as a binder and the non-linearity in the voltage-current characteristic is determined by the voltage dependency of the resistance between grains or through the grain boundaries so that the C-value is adjustable by changing the thickness of the varistor element which is a function of the number of grain boundaries in the direction of the current. In the varistor elements for low voltage use, however, the number of grain boundaries must be so small owing to the relatively large C-value per grain boundary that the breakdown voltage is also disadvantageously decreased. In addition, silicon carbide-based varistor elements have a relatively small non-linearity index .alpha. of 3 to 7 and, moreover, difficulties are brought about due to the extreme hardness of the silicon carbide particles in the rapid wearing of the metal molds for shaping and in the unsatisfactory precision in the dimensions of shaped bodies.
On the other hand, varistor elements of selenium or cuprous oxide are unsatisfactory from the standpoint of practical use because their non-linearity indexes are only 2 to 3 and they cannot be used with large limiting voltages.
Further, the non-linearity index of zinc oxide-based varistor elements is large enough to be in the range of 10 to 50 along with fine particle size zinc oxide of about 10 .mu.m or smaller and they can be advantageously used at a voltage varied in the range from 10 to 1000 volts. However, zinc oxide-based varistor elements are not free from deterioration of the nonlinearity characteristics with the lapse of time and they are also defective in the high cost of their production owing to the complicated manufacturing process and diversity of the necessary additive components.
Thus, there has been a strong demand in the electric industries for varistor elements, in which the non-linearity is obtained with the material per se little dependent on the boundary phenomena and any desired C-values can readily be obtained by chainging the thickness of the element in the direction of current without modifying the value of non-linearity index. Further there has been a demand for varistor elements having a larger non-linearity index .alpha. than silicon carbide-based varistors to be applicable to a wide variety of fields with low cost.
For example, a material for varistor elements is proposed in Japanese Patent Publication No. 53-11075 which is a sintered body composed of titanium dioxide admixed with 0.1 to 3% by moles of niobium oxide and 0.05 to 1.0% by moles of bismuth oxide. This material has a larger non-linearity index .alpha. than silicon carbidebased varistors and selenium or cuprous oxide varistors, and presents an advantage that a desired C-value can be obtained without changing the value of .alpha.. One of the problems in this type of varistor elements is the uncontrollable variation in the performance of the products due to the difficulty in obtaining uniform distribution of the niobium oxide and bismuth oxide in the powder mixture to be subjected to sintering.
Further, varistor elements are proposed in Japanese Patent Publication No. 52-235 and U.S. Pat. No. 3,715,701 with a sintered body composed of titanium dioxide admixed with 0.005 to 0.1 mole of bismuth oxide and 0.001 to 0.05 mole of antimony oxide per mole of titanium dioxide. These varistor elements are also not free from the same problem of poor dispersion as in the titanium dioxide elements admixed with niobium oxide and bismuth oxide.
Turning now to the problem of noise suppression in rotatory machines, especially, in miniature motors, all of precision instruments of compact size utilizing miniature motors are subject to the disturbance by the noise generated in the motors with the sparking phenomenon between the commutator and the brush. To explain it, commutators in electric motors are shaped in a cylindrical form as composed of a plurality of commutator segments assembled with regular intervals of insulating layers. Therefore, the brush in contact with the rotating commutator moves from one segment to the next one jumping on the surface of the commutator over the insulating layer producing sparks by the spike voltage which is due to the large self-inductance inherent to a rotor constructed with a coil wound around a magnetic body. This phenomenon of sparking causes the electric noise generated in motors and, in addition, is undesirable due to the shortened life of the motor by the accelerated wearing of the commutator and the brush.
This spike voltage for sparking is a bipolar oscillating voltage with peak heights of as high as 20 to 50 times of the line voltage of the motor with a high frequency component of 2 to 5 MHz lasting for about 100 .mu.seconds. In order to eliminate the noise voltage with such a high frequency component and to stabilize the operation of the instrument, a noise suppressor is indispensable having a non-linearity in the voltage-current characteristic as large as possible working at a voltage of 3 to 30 volts and capable of absorbing the high frequency component in the noise voltage whereby to decrease the noise voltage to a level of the line voltage of the motor. Hitherto known noise suppressors utilizes various principles and a variety of materials such as varistor elements but none of the prior art materials are unsatisfactory in several aspects.